THE INFLUENCE OF GRAVITY 



83 



in this region may be due to a thickening of the cortex, a point 

 which needs further investigation. 



4. Influence of Light. — In stems with a large apical leaf the 

 geotropic curvature takes place also in the dark although less 

 rapidly than in the light. In this case the material formed 

 previously and stored in the leaf may be utilized, since the geo- 

 tropic curvature takes place very rapidly. On account of the 

 fact that light plays less of a role in the case of geotropic curva- 

 ture of the stem than in the case of regeneration of shoots or roots 

 the suspicion arose that the curvature was only due to a collection 

 of water on the lower side of the stem. To test this idea experi- 



FiG. 63. — Geotropic curvature occurs also in stems put under water, though the 

 degree of geotropic curvature seems to be diminished. 



ments were made with stems partly under water. It was found 

 that when the bending part of the stem was entirely under water 

 (Fig. 63), the curvature nevertheless occurred though it may have 

 been diminished. This suggests that the curvature was due to 

 the solutes but not exclusively to the water of the sap sent out 

 by the apical leaf into the stem. 



The theory of geotropic curvature of the stem of Bryophyllum 

 calycinum when put in a horizontal position is then as follows : 

 This curvature is determined by the excess of longitudinal growth 

 of the cortical layers on the under side of the stem and this 

 excess of growth increases with the mass of an apical leaf attached 

 to the stem. The geotropic curvature seems therefore to be a 



